1. Field of the Invention
The subject matter of the invention is an exhaust gas aftertreatment system for lean-burn internal combustion engines such as diesel engines and Otto engines with direct injection wherein the system has a catalyzer for oxidation of nitrogen monoxide installed in the exhaust gas train.
2. Description of the Related Art
In order to adhere to the legally prescribed limits on exhaust gas, virtually all lean-burn internal combustion engines have been outfitted in the meantime with catalytic aftertreatment systems such as:                NOX storage catalyzers        SCR catalyzers, or        particulate filters.        
In all of these systems, nitrogen dioxide (NO2) is an important component in the reactions taking place in the aftertreatment system.
The powerful oxidizing agent NO2 is formed at catalyzers, usually containing platinum, for oxidizing nitrogen monoxide (hereinafter: NO oxidation catalyzers) by means of oxygen which is contained in the exhaust gas and formed from the nitrogen monoxide emitted by the engine.2NO+O22NO2  (1)
The problem with these NO oxidation catalyzers is that the maximum NO2 proportions that can be achieved are limited thermodynamically at high temperatures. As a result, in contrast to other exhaust gas catalyzers, the desired conversions will decrease again at high temperatures after an increase at low temperatures and there will not be a pronounced plateau-like conversion maximum.
The SCR (Selective Catalytic Reduction) method is an established means for reducing nitrogen oxides. SCR catalyzers have been used for many years in the energy industry and more recently also in internal combustion engines. A detailed exposition of these methods is given in DE 34 28 232 A1. V2O5-containing mixed oxides, e.g., in the form of V2O5/WO3/TiO2, can be used as SCR catalysts. V2O5 proportions typically range between 0.2% and 3%.
In practical applications, ammonia or compounds which split off ammonia such as urea or ammonia formiate are used in solid form or in solution as reductants. One mole of ammonia is needed to convert one mole of nitrogen monoxide.4NO+4NH3+O24N2+6H2O  (2)
For the decomposition of the reductant, the exhaust gas temperature, particularly after the internal combustion engine is started or when the internal combustion engine is operated in the lower output range, is too low to generate ammonia without the occurrence of problematic byproducts.
In connection with the decomposition of urea ((NH2)2CO) in ammonia (NH3), it is known that this takes place under optimal conditions (temperatures above 350° C.) in two steps. First, thermolysis, i.e., the thermal decomposition, of urea takes place according to the following reaction:(NH2)2CONH3+HNCO  (3)
This is followed by hydrolysis, that is, the catalytic decomposition, of isocyanic acid (HNCO) into ammonia (NH3) and carbon dioxide (CO2) according to the following reaction:HNCO+H2ONH3+CO2  (4)
When the reductant is in aqueous form such as in a eutectic urea solution (trade name: AdBlue), for example, this water must also evaporate prior to and during the actual thermolysis and hydrolysis.
If the temperatures during the above-mentioned reaction (3) and (4) are below 350° C. or if heating is only gradual, it is known from DE 40 38 054 A1 that chiefly solid, infusible cyanuric acid is formed through trimerization of the isocyanic acid formed in (5):
                    3        ⁢                                  ⁢        HNCO        ⁢                                  ⁢                                                                           <                                  350                  ⁢                  °                  ⁢                                                                          ⁢                                      C                    .                    ⟶                                                                        ⁢                                                                      ⟵                          >                              350                ⁢                °                ⁢                                                                  ⁢                                  C                  .                                                                    ⁢                                  ⁢                              (            HNCO            )                    3                                    (        5        )            leading to clogging of the SCR catalyzer downstream. As is stated in DE 40 38 054, cited above, this problem can be remedied by guiding the exhaust gas flow charged with the reductant through a hydrolysis catalyzer. Thus, the exhaust gas temperature at which a quantitative hydrolysis is first possible can be brought down to 160° C. The construction and composition of a corresponding catalyzer is likewise described in the above-cited publication as is the construction and operation of a SCR catalyzer system outfitted with a hydrolysis catalyzer.
When a platinum-containing NO oxidation catalyzer for forming NO2 is positioned in front of the SCR catalyzers2NO+O22NO2  (1)the SCR reaction can be substantially accelerated and the low temperature activity is noticeably increased.NO+2NH3+NO22N2+3H2O  (6)
In this connection, it must be ensured that the NO2 proportion of the total nitrogen oxides does not exceed 50% because this would lead to a decrease in the NOx conversion.
Nitrogen oxide reduction using the SCR method in internal combustion engines operating in vehicles is difficult because of the changing operating conditions, which makes it difficult to apportion the reductant in terms of quantity. On the one hand, the highest possible conversion of nitrogen oxides must be achieved; but on the other hand emission of unspent ammonia must be prevented. This problem is often solved by using an ammonia blocking catalyzer downstream of the SCR catalyzer to convert the excess ammonia to nitrogen and water vapor. Further, the use of V2O5 as active material for the SCR catalyzer leads to problems when the exhaust gas temperature at the SCR catalyzer is above 650° C. because the V2O5 is then sublimated.
Particle separators, as they are called, or particulate filters are used in power plants and in vehicles to minimize fine particles. A typical arrangement with particle separators for use in vehicles is described, for example, in EP 1 072 765 A1. Arrangements of this kind differ from those using particulate filters in that the diameter of the channels in the particle separator is substantially greater than the diameter of the largest occurring particle, while the diameter of the filter channels in particulate filters is in the range of the diameter of the particles. Due to this difference, particulate filters are prone to clogging, which increases the exhaust gas back pressure and reduces engine performance. An arrangement and a method with particulate filters are shown in U.S. Pat. No. 4,902,487. A distinguishing feature of the two above-mentioned arrangements and methods is that the oxidation catalyzer—usually a catalyzer with platinum as active material—arranged upstream of the particle separator or particulate filter oxidizes the nitrogen monoxide in the exhaust gas by means of the residual oxygen that is also contained to form nitrogen dioxide which is converted in turn in the particle separator or particulate filter with the carbon particles to form CO, CO2, N2 and NO. In this way, a continuous removal of the deposited solids particles is carried out. Accordingly, regeneration cycles that must be carried out uneconomically in other arrangements are dispensed with.2NO2+C2NO+CO2  (7)NO2+CNO+CO  (8)2C+2NO2N2+2CO2  (9)
In order to meet future exhaust gas regulations, it will be necessary to use arrangements for reducing nitrogen oxide emissions and arrangements for reducing fine particles emissions at the same time. Various arrangements and methods are already known for this purpose.
U.S. Pat. No. 6,928,806 describes an arrangement including an oxidation catalyzer, a SCR catalyzer arranged downstream of the latter in the exhaust gas flow, and a particulate filter which is arranged downstream of the latter in the exhaust gas flow. The reductant for the selective catalytic reaction taking place in the SCR catalyzer is fed back immediately in front of the SCR catalyzer by a urea injection device that is controlled as a function of the operating parameters of the internal combustion engine. A disadvantage in this arrangement is that the nitrogen dioxide generated in the oxidation catalyzer is substantially completely consumed by the selective catalytic reduction in the SCR catalyzer; that is, it is no longer available for the conversion of the solids particles that have accumulated in the particulate filter arranged downstream. Therefore, the regeneration of the particulate filter must be carried out uneconomically through cyclical heating of the exhaust gas flow by enriching the exhaust gas flow with unconsumed hydrocarbons. This is accomplished either by enriching the combustion mixture or by injecting fuel in front of the particulate filter. On the one hand, an arrangement of this kind for regenerating the particulate filter is elaborate and therefore expensive. On the other hand, the cyclical regeneration of the particulate filter situated at the end of the arrangement produces harmful substances again which can no longer be removed from the exhaust gas.
U.S. Pat. No. 6,805,849 discloses another combination of a particulate filter and an arrangement for selective catalytic reduction. The arrangement described therein includes an oxidation catalyzer in the exhaust gas flow which increases the proportion of nitrogen dioxide in the exhaust gas, a solids filter arranged downstream, a reservoir for the reducing liquid, an injection device for the reducing liquid which is arranged behind the solids filter, and an SCR catalyzer downstream of the latter in the exhaust gas flow.
When an NOx storage catalyzer is used, the combustion changes constantly between overstoichiometric combustion and substoichiometric combustion. In the lean operating phases, the nitrogen oxides are stored in the form of nitrates which are reduced to nitrogen in the rich operating phases by means of carbon monoxide and hydrocarbons. The storage in the form of nitrate proceeds by way of NO2 which accumulates in the form of nitrate on the barium or calcium storage components.
As was already mentioned, the NO2 needed for the reactions described above is formed at NO oxidation catalyzers usually containing platinum. In actual engine operation, however, sulfurization of the NO oxidation catalyzers due to sulfur contained in the fuel and/or in the engine oil poses a problem. Owing to the combustion, SO2 is formed from this sulfur and is oxidized at the NO oxidation catalyzers downstream to form SO3.S+O2SO2  (10)2SO2+O2SO3  (11)
In this connection, it has been shown that the amount of SO3 which is formed and the amount of NO2 which is formed are directly related; this means that a catalyzer forming large amounts of NO2 generates large amounts of SO3 at the same time.
This SO3 forms sulfates with the metal-containing catalyzer washcoat or sulfuric acid with water which is adsorbed on the surface.H2O+SO3H2SO4  (12)
Both lead to a covering of the active centers of the catalyzer and, therefore, to decreased activity. A regeneration of the catalyzers can be carried out by increasing the exhaust gas temperatures to greater than 500° C., but this temperature is rarely achieved in normal vehicle operation, especially when an exhaust gas turbocharger is used. Further, the active temperature increase is usually connected to an increase in fuel consumption.